I want to test a center filter on a wideangle lens that the center filter is not specifically designed for. What is the best way to check for uneven illumination? Thanks & best regards, Åke

-- Ake Vinberg (ake@vinberg.nu), April 01, 2002

Answers

The best way, in my opinion, is to photograph an evenly lit subject, e.g. a gray card or something likewise. (Maybe something bigger, but it should render an even gray negative.)
Then you should give the negative a plus development, as this enhances the effect of uneven illumination. Then on to the enlarger, which I hope that you have already checked for uneven illumination. (If not, adjust the lamp to the best of your abilites, and print the full negative area, without a negative in the holder, on the hardest contrast paper that you can find. You should adjust the exposure time to print a middle gray, as any differenties are easier to spot that way.)
Now print the whole negative on a very high contrast paper. Adjust the exposure time to get a middle gray value. Compare the result with the one that you get without a negative in the enlarger.

-- Björn Nilsson (b.w.nilsson@telia.com), April 01, 2002.

Schneider recently informed me that they never made a center filter for the 121 Super Angulon that I have. I wonder if the one made for the 120 would work? Or any 82mm Heliopan? Or nothing?

Bill

-- Bill Marsh (redcloud54@earthlink.net), April 01, 2002.

Greetings,

In addition to what Björn said, you can eliminate the effects of enlarger error, by using a densitometer to read the density reading on the negative/transparency. If you don't own a densitometer, you may be able to have the density read at a commercial lab, university, or even a hospital X-ray deparment.

Regards,

-- Pete Caluori (pcaluori@hotmail.com), April 01, 2002.

Focus the lens on infinity, then point the camera at a uniform object which is uniformally illuminated. The reason for focusing on infinity is that the unevenness will be worse at this distance because the lens is closest to the film. Repointing at a closer object is better because it is very hard to find a large distant object that is uniform in brightness. The close object will be out-of-focus, thereby smearing away any small-scale brightness variations. Be sure to avoid shadowing part of the object being photographed.

Use the most contrasty material that you plan to use, e.g. B+W developed to N+1, or N+2 if you do that, or color transparency.

I suggest taking a test photo with and without the center filter so you can see what difference it makes. The manufacturers typically do not design center filters to result in perfectly even illumination because this would require long exposures and because it isn't necessary. Following this approach, don't reject a center filter if it doesn't yield perfectly even illumination--the goal is to reduce the uneveness to an acceptable level. Real scences almost always have some variation in brightness, and minor variations can be taken care by burning in while enlarging.

When I did this, I judged the negatives by looking at them and comparing the variation in density to the density range of a typical negative. Going on to printing the negative would make it a full end-to-end test. If you want to test your enlarger, you could make another test negative in the same manner, using your longest focal length taking lens.

-- Michael Briggs (MichaelBriggs@EarthLink.net), April 01, 2002.

Thanks for all replies. Since I do not have access to a darkroom I will have to study the film directly, or scan and evaluate in Photoshop (which is the workflow I normally use). This probably means that color transparency film is more appropriate.

It sounds like finding an evenly illuminated subject is the key. Here is what I will do:

- Rig up a piece of white cardboard paper in sunlight, taped on a window for flatness

- Set up the camera as close as possible to the paper without acutally shadowing it,

- Focus on infinity,

- Expose for medium gray on color transparency film to also check for color casts,

- Expose with and without center filter for comparison.

- Expose center as well as full shift, past image circle boundary if possible.

What I want to avoid is a lens/filter combination that creates a darker center, or a bright or dark halo because of dark spot size mismatch. I can accept if the CF does not fully compensate. The CF in question is for a 5.6/65 Super Angulon, and the lenses are a 5.6/47 Super Angulon XL and a 6.8/90 Grandagon MC, all with 67mm thread.

I will post my results in this thread.

Åke

-- Ake Vinberg (ake@vinberg.nu), April 02, 2002.

I was going to suggest the Photoshop solution to you - simply scan a transparency of an evenly illuminated solid color, and check the various K values in PS. This would give you an accurate reading on the existing light falloff, then do the same check with the filter in place.

-- Michael Mahoney (mike.mahoney@nf.sympatico.ca), April 04, 2002.